Let's take 2 examples:
The cashew nut, although a nut in the culinary sense is actually a seed in the botanical sense. The seed is surrounded by a double shell containing some serious poisons. This is why you will never find a cashew nut in its shell at the supermarket.
The castor bean is infamous for the extraction of Ricin from it. This is one of the most potent poisons known to man and 500 micrograms (the size of a grain of salt) will kill you. Enough said really.